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- this data is likely to be accessed soon (_could_ lead to a
prefetch)
- this member field is hot/cold (iirc, Class members may be
re-ordered) (UDA)

- this pointer is aligned to N bytes
- this pointer doesn't alias with this pointer
There are also loop hints:
- this loop has no loop dependency, go wild auto-vectorizer!
- this loop is better unrolled by N

These hints are all dangerous. Manual prefetching, manual branch
hints, forced unrolling count: I've seen those 3 hints reduce
performance by being worse than the processor/compiler.